I don’t know about tools for writing custom generators, but for the specific case of inst distribution there is a ticket in Jira, https://dev.clojure.org/jira/browse/CLJ-2179. There Gary Fredericks recommends generating the components year, month, day, etc. separately and combining them with gen/fmap. test.check does have generators with uniform distribution so presumably the idea is to create a custom generator for bounded insts and attach it to a spec (I haven’t tried it though).

@alanmarazzi sure, the project is at https://github.com/glts/spelunc. The problem with Gorilla REPL is that you currently have to build your own snapshot to make it work with Clojure 1.9 (as far as I know). And Huri needs a working R installation, the huri.plot stuff is really just a Clojure facade of the ggplot2 library.